In a wireless apparatus, such as a mobile phone and the like, a transmitting circuit and a receiving circuit are connected to a common antenna through a high-frequency semiconductor switch (hereinafter, referred to as a semiconductor switch) which switches the connection to the common antenna between the transmitting and receiving circuits. The semiconductor switch includes a Metal-oxide-semiconductor field-effect transistor (MOSFET) formed on a Silicon on Insulator (SOI) substrate. In such a semiconductor switch, it is desirable to reduce insertion loss in a high frequency region.